As legend has it, Guthrie wrote the song in response to the hugely popular Irving Berlin song, "God Bless America." He was so tired of hearing the song on the radio and the blatant jingoism it promoted. Guthrie had seen much of America by this time, had experienced the Dust Bowl exodus of the 1930s and the racism and classism that followed emigrant workers and the blue collar working boys around, as they searched for work during the Great Depression.
With all of that in mind, Woody Guthrie wrote a new song of patriotism, with the refrain line, "God blessed America for me."
The song includes rather poignant lyrics that were considered subversive in their time. Lyrics to the song tackle the definition of liberty, individual rights and property ownership.
The original version of the song included two controversial verses that were edited out in ocnsequent versions and recordings, although, if you see Arlo Guthrie in concert these days, he'll generally include these two verses in his performances.
Visit WoodyGuthrie.org for the full lyrics to "This Land is Your Land"